When clicking the Manage Attendees button on a Medical Event, the page does not render the Account Lists.
This problem occurs if the user does not have Field Level Security (FLS) access to the field Country_vod__c in the Address_vod__c object.
Not having this field visible prevents the building of a list of Person_Account for Events.
Provide Field Level Security (FLS) access to the Country_vod__c field in the Address_vod__c object.
CRM Help Documentation: Adding Attendees to an Event